This invention relates to an automatic focusing adjustor device for use in a camera having a distance meter of the double image coincidence type. In this type of system, a distance scanning is performed over the object to be photographed, and an object lens is moved to a desired position corresponding to the object distance by applying the focusing sensor signal sensed by an automatic focusing sensor device for automatically sensing the focusing position. The present invention further relates to a focusing adjustment when the focusing sensor signal is not be produced.
At first, referring to FIGS. 1 and 2, the principle of operation of the automatic focusing sensor device to which the present invention may be applied will be described.
In FIG. 1 is shown a preferred embodiment of a distance measurement system and a focusing sensor circuit of an automatic focusing sensor device and in FIG. 2 is shown waveforms of each of the major signals which will be described with reference to FIG. 1.
Referring at first to FIG. 1 in which an automatic-focusing sensor device is illustrated, reference numeral 1 indicates a fixed mirror and reference numeral 2 indicates a movable mirror.
The movable mirror 2 may be rotated in an angular range extending from a position corresponding to the nearest distance (the nearest distance where photographing may be performed by a camera etc.) to the infinite distance (45.degree. with respect to an optical axis). On the other hand, the fixed mirror 1 is set at an angle of 45.degree. with respect to the optical axis. When the movable mirror 2 starts to rotate, as shown by the curved arrow, from the nearest position (position n indicated by dash-lines in the drawing) toward the infinite position (a position of .infin. indicated by a solid line in the drawing) in a clockwise direction, light from the object incident against each of the mirrors is reflected by a prism 3, passes through the light collecting lenses 4 and 5 and finally is received by the photoelectric transfer elements 6 and 7. Each of the photoelectric transfer elements 6 and 7 is composed of finely divided photoelectric elements and each of their outputs is fed to a well-known focusing sensor module 8. The relative relation of each of the beams of incident light represented by the photoelectric element output signals is calculated to generate an output signal .SIGMA.OUT, a so-called relative signal. The output from the module 8 is connected to a peak sensor circuit comprised of a comparator 10, a transmission gate 9 and a capacitor Cp. As shown in the drawing, the output signal .SIGMA.OUT applied to the non-inverting input (+) of the comparator 10 and to the inverting input (-) via transmission gate 9. To the inverted input of the comparator is further connected a capacitor Cp and the output terminal of the comparator 10 is connected to a control terminal of the transmission gate 9.
Operation of the comparator 10 is as follows. Off-set adjustment is set such that when the same value voltage is applied to the inverting input and the non-inverting input, its output PH will be "H". The transmission gate is set such that the same conducts when a "H" level signal is applied to the control terminal and in turn is shut off when a "L" level signal is applied to the control terminal.
Referring now to FIG. 2, the focusing process from rotation of the movable mirror 2 to a time when the focusing sensor signal is sensed will be described. FIG. 2A illustrates a curve indicating operation of the movable mirror 2, FIG. 2B illustrates the waveform of the output signal .SIGMA.OUT of the module 8 and FIG. 2C illustrates the waveform of the output signal PH of the comparator 10.
When it is assumed that some variations of the output signal .SIGMA.OUT obtained by a rotation of the movable mirror 2 are shown by the waveform in FIG. 2B, a slope or inclination of .SIGMA.OUT signal has a positive value when the movable mirror 2 starts to rotate from the nearest position to reach D.sub.1, the comparator "+" input will have a higher value than the "-" input (.SIGMA.OUT signal is slightly delayed by the capacitor Cp before being applied to the "-" terminal), its output PH will become "H" level to cause the transmission gate 9 to be conductive. Since .SIGMA.OUT will have a negative slope when the movable mirror 2 is moved past a position of D.sub.1, the voltage applied to the "+" input to the comparator 10 is decreased lower than that applied to the "-" input with the result that its output PH is reversed to the "L" level and at the same time the transmission gate 9 is shut off. Thus, in the capacitor Cp is stored focusing sensor module output signal (a cooperative signal) which is obtained when the movable mirror 2 is positioned at D.sub.1. When the movable mirror 2 proceeds to rotate beyond the position of D.sub.2, the output signal .SIGMA.OUT is a higher voltage than that stored by the capacitor Cp for the case of D.sub.1, so that the comparator output PH becomes "H" level again. Further when the movable mirror 2 continues to rotate to reach a position of D.sub.3 and slightly beyond that position, the output signal PH becomes "L" level again. Further even if the movable mirror 2 proceeds to rotate up to a position of ".infin.", the .SIGMA.OUT signal will not reach the value of the output signal obtained at a position of D.sub.3, so that the output signal PH from the comparator remains "L" level. At this time, the output signal .SIGMA.OUT will reach a maximum peak value when the distribution of light incident on the fixed mirror and the distribution of light incident on the movable mirror coincide with each other during rotation of the movable mirror. The signal PH will be changed from "H" to "L" at a maximum peak value of the .SIGMA.OUT signal and produce a focusing sensor signal. The time when this signal is generated corresponds to the distance D.sub.3 and this shows that the object is placed at a position of the distance D.sub.3. In other word, this fact shows that a position of the movable mirror within the range from its initial position at the start of scanning to a mirror position corresponding to the final "L" level of PH signal corresponds to the object distance.
On the basis of a focusing sensor signal obtained in such a manner as described above, the object lens will be adjusted for focusing it at a desired position.
However, in such a device as described above, when the object exhibits an extremely low contrast the output signal .SIGMA.OUT will become flat shaped and will not show any peak value, so that the focusing sensor signal may not be obtained. In such a case, it has already been proposed to set a focusing adjustment to a specified position, for example, the infinite position or a normal focusing position.
However, at present, it is apparent that under some conditions the object shows an extremely low contrast, for example in such a case as when the weather shows a fine condition or the object is placed at a distant position. When the object is placed at a distant position, an expected range covered by the finely divided photoelectric elements illustrated in FIG. 1 will be expanded, and thus a brightness of incident light applied to each of the photoelectric elements will be equalized with each other, resulting in only a flat shaped .SIGMA.OUT signal being obtained.
To the contrary, when the object shows a low brightness, a low contrast always results and sometimes a peak value may not be obtained. Photographing condition when the object has a low brightness may frequently be found in such as case as when persons are to be photographed inside a room, i.e. photographing at a relatively short distance will be performed.